What materials are used for the heating elements in belt-type normalizing furnaces?

The material of the heating elements in a mesh-belt normalizing furnace must be selected based on the furnace temperature range, the internal atmosphere, and the required service life. The mainstream materials fall into two major categories: metal alloys and non-metallic ceramics. The specific characteristics and applicable applications are as follows:

I. Metal Alloy Heating Elements
These components exhibit excellent formability and convenient wiring, making them well suited for medium- and low-temperature (≤1200°C) operating conditions in belt-type normalizing furnaces, and they are widely used in industrial furnace applications.
Iron-chromium-aluminum alloys (such as 0Cr25Al5 and 0Cr21Al6Nb)
Key characteristics: excellent high-temperature resistance, superior oxidation resistance, and high resistivity; can be fabricated into strip, wire, and tubular components. It exhibits minimal deformation at elevated temperatures but significant embrittlement at low temperatures, so impact should be avoided during handling and installation.
Applicable scenarios: the medium-to-high temperature section (800–1200°C) of belt-type normalizing furnaces, suitable for normalizing in air or a neutral atmosphere, such as the normalizing treatment of carbon steel and low-alloy steel workpieces.
Advantages and disadvantages: high cost-effectiveness and relatively long service life (1–2 years under normal operating conditions); however, it exhibits poor resistance to corrosive atmospheres such as sulfur- and chlorine-containing environments, making it prone to corrosion and failure in sulfur-containing furnace gases.

Nickel-chromium alloys (such as Cr20Ni80 and Cr15Ni60)
Key features: high-temperature resistance, excellent ductility, low susceptibility to brittle fracture, and ease of machining and forming; resistivity is lower than that of iron-chromium-aluminum alloys, resulting in a larger cross-sectional area for the element at the same power rating.
Applicable scenarios: the medium- and low-temperature sections of belt-type normalizing furnaces, particularly for applications requiring high component toughness or operating under mildly corrosive atmospheres within the furnace, such as low-temperature normalizing of nonferrous metal workpieces.
Advantages and disadvantages: excellent toughness and strong resistance to thermal shock; however, it is more expensive than iron-chromium-aluminum, has lower high-temperature strength, and is prone to creep deformation when operated continuously at temperatures above 1000°C.

II. Non-metallic Ceramic Heating Elements (for High-Temperature Normalizing)
These components exhibit excellent high-temperature resistance and are well suited for the high-temperature (above 1000°C) operating conditions of belt-type normalizing furnaces; however, they are highly brittle and demand stringent installation and maintenance procedures.
Molybdenum Silicide Rod (MoSi₂)
Key features: high-temperature resistance; at elevated temperatures, a SiO₂ glassy surface film forms, providing excellent oxidation resistance; resistivity increases with rising temperature, requiring a dedicated voltage-regulating power supply.
Applicable scenarios: the high-temperature section (1200–1600°C) of belt-type normalizing furnaces, suitable for high-temperature normalizing of stainless steels and high-alloy steels, as well as heat treatment under vacuum or in an inert atmosphere.
Advantages and disadvantages: excellent high-temperature resistance and relatively long service life (6–12 months under high-temperature operating conditions); however, it exhibits severe low-temperature brittleness, making it prone to fracture at room temperature. Therefore, during start-up and shut-down, temperature changes must be carried out slowly to avoid rapid heating or cooling.

Silicon Carbide Rod (SiC)
Key features: high-temperature resistance, excellent thermal conductivity, superior thermal shock resistance compared with silicon-molybdenum rods; stable resistivity, allowing direct operation from standard power supplies.
Applicable scenarios: the medium-to-high temperature section (1000–1400°C) of belt-type normalizing furnaces, suitable for normalizing operations on ceramic and refractory workpieces, or for service conditions in which a mild acidic atmosphere exists within the furnace.
Advantages and disadvantages: good thermal shock resistance and lower cost than silicon-molybdenum rods; however, it is prone to oxidation at high temperatures, its service life is highly dependent on the furnace atmosphere, and prolonged operation above 1300°C leads to accelerated aging.

III. Material Selection Comparison Table
Iron-chromium-aluminum alloy: 1250°C, 800–1200°C; high cost-effectiveness and good oxidation resistance, but significant low-temperature brittleness and poor resistance to corrosive atmospheres.
Nickel-chromium alloy: 1100°C, 400–1000°C; good toughness and resistance to thermal shock; prone to creep at high temperatures; relatively expensive.
Silicon-molybdenum rods: 700°C, 1200–1600°C; high-temperature resistant and suitable for high-temperature normalizing; prone to breakage at low temperatures and require slow heating and cooling.
Silicon carbide rods: 1450°C, 1000–1400°C; good thermal shock resistance and low power requirements; prone to oxidation at high temperatures; service life is affected by the atmosphere.